Progress of the advanced information society is remarkable in recent years. The multimedia, with which various forms of information can be handled, are rapidly popularized. A magnetic recording apparatus, which is installed to a computer or the like, is known as one of the multimedia. At present, the development is advanced in order that the magnetic recording apparatus is miniaturized while improving the recording density.
In order to realize the high density recording by the magnetic recording apparatus, for example, it is demanded that (1) the distance between the magnetic disk and the magnetic head is narrowed, (2), the coercivity of the magnetic recording medium is increased, (3) the method for processing the signal is executed at a high speed, and (4) a medium, which suffers from less thermal fluctuation, is developed.
In order to realize the high density recording especially at a surface recording density exceeding 50 Gbits/inch2 (about 7.75 Gbits/cm2), it is necessary that the unit (magnetic cluster), in which the magnetization reversal occurs in the magnetic layer during the recording and the erasing, is further decreased, and the distribution thereof is precisely controlled. In order to decrease the magnetic cluster, it is necessary that the crystal grains, which constitute the magnetic layer, are made fine and minute, or the number of crystal grains for constituting the magnetic cluster is decreased.
In view of the reduction of the thermal fluctuation, it is also important to reduce the dispersion of grain diameters when the crystal grains are made fine and minute. The thermal fluctuation tends to occur in the case of the minute crystal grains. Therefore, it is necessary that the crystal grain diameters are controlled to be not less than a certain size. However, if the crystal grain diameters are unnecessarily increased, and the crystal grains become coarse, then the noise increases in some cases when information recorded at a high density is reproduced. Therefore, it has been necessary to strictly control the grain sizes of the crystal grains and the distribution thereof. As a trial to realize the above, it has been suggested to provide a seed film between a substrate and a magnetic layer as disclosed, for example, in U.S. Pat. No. 4,652,499.
However, the method, in which the magnetic layer is provided on the substrate with the seed film intervening therebetween, has had a limit to control the crystal grain diameters and the distribution thereof in the magnetic layer. It has been difficult to sufficiently suppress the noise and the thermal fluctuation generated from the medium, for example, even when information is recorded at a recording density exceeding 50 Gbits/inch2 (about 7.75 Gbits/cm2).
For example, even when the material for the seed film, the film formation condition, and the structure of the seed film are adjusted in order to obtain crystal grains of the magnetic layer having grain diameters of about 10 nm, the grain diameter distribution of the obtained crystal grains is a Gaussian distribution having wide lower slopes, in which coarse grains having about double sizes of 10 nm and minute grains inversely having about half sizes of 10 nm are considerably present in a mixed manner. The crystal grains, which have grain diameters larger than the average of those of the crystal grains, cause the increase of noise during the recording/reproduction. On the other hand, the crystal grains, which have grain diameters smaller than the average, increase the thermal fluctuation during the recording/reproduction.
In order to suppress the occurrence of the thermal fluctuation as described above and allow the minute crystal grains to exist stably, it is enough that the coercivity of the magnetic layer is raised. However, if the coercivity of the magnetic layer is raised, it is necessary to strengthen the magnetic field to be generated by the magnetic head. In such a situation, it has been difficult to record information by using the existing magnetic head in some cases. Accordingly, a method has been suggested, in which the magnetic layer is heated by irradiation with a light beam to temporarily lower the coercivity during the recording of information, and the information is recorded by applying the magnetic field to an area in which the coercivity has been lowered. Such a recording system is called “thermal assist magnetic recording”. Even in the case of the magnetic recording medium in which the coercivity of the magnetic layer is enhanced, information can be recorded even when the existing magnetic head is used, because the coercivity of the recording layer is decreased by effecting the heating by means of the irradiation with the light beam during the recording of information. However, in the case of the thermal assist magnetic recording system, it is necessary that the area which is irradiated with the light beam and the area to which the magnetic field is applied are correctly coincident with each other. Therefore, it is necessary to position the magnetic head and the optical head highly accurately. As a result, a problem arises such that the structure of the recording head is complicated and the production cost is expensive.
The present invention has been made taking the foregoing circumstances into consideration. A first object of the present invention is to provide a novel recording method for recording information at a super high density on an information-recording medium.
A second object of the present invention is to provide an information-recording apparatus which is excellent in thermal stability and which has high reliability.
A third object of the present invention is to provide an information-recording medium which makes it possible to record information at a super high recording density of not less than 50 Gbits/inch2 (about 7.75 Gbits/cm2) in a magnetic film for information recording.